http://dx.doi.org/10.4067/S0716-07202001000100007

Gastrointestinal nematode infection in ewes raised in an arid zone of Venezuela

The necropsy of a total 72 ewes coming from the Venezuelan arid zone (State of Falcón) was carried out as follow: 6 ewes per month; 30 during rainfall period and 42 during the dry period. The following nematode species were identified: Haemonchus contortus, Trichostrongylus axei, T. colubriformis, Cooperia fuelleborni, C. pectinata, C. curticei, C. punctata, Trichuris ovis, Oesophagostomum columbianum, Skrjabinema ovis and Bunostomum trigonocephalum. The values of the Bulla's diversity and evenness indices for the infracommunity of strongylids of ewes during the dry and rainfall periods, evidenced a low diversity and an unequal numerical repartition of species within the community. The Sorensen similarity coefficient (Css = 77%) showed a high similarity between the parasite species composition of communities from both periods. However no relationship between the structural patterns of the community was observed (rs = -0.08).

Gastrointestinal strongylidosis is a major pro-blem for the development of small ruminants production in Venezuela.1 Helminth parasites have effects on the growth rate of lambs and on milk and wool production in adult sheep.2 In field conditions, mixed infections by several gastrointestinal nematode species are usually encountered3-1, 4 and the severity of the losses are in relation to the species present2and the magnitude of the parasite worm burden.5Ho-wever it appears that a low worm burden could be of pathogenic significance in arid and semiarid regions.6

The aim of the present study was to evaluate the effects of seasonal rainfall extremes on the gastrointestinal nematode infracommunities in ewes infected under natural conditions from an arid zone of Venezuela.

MATERIALS AND METHODS

Study Area: The locality of Pedregal (70° 7' N, 11° 2'W) and 158 meters above sea level, at Falcón State where the present study was conducted has a low rainfall average (X = 34.1 mm/month) with an average monthly temperature of 27.9°C. The dryer months (< 34.1 mm) are from December to March (X = 3.7 mm) and from June to August (X = 25.7 mm). During April, May, September, October and November the highest values of rainfall occurs (X = 63.5 mm). To calculate these averages, 5 years of rainfall data were used (1990-1994).

Parasitological Methods: The number of ewes examined during the dry and rainy periods were 42 and 30 respectively (6 ewes/month). The alimentary tracts were collected from slaughterhouse at Coro (State of Falcon, Venezuela). Viscera were frozen and later exa-mined for helminths. Worms were recovered7and identified with the aid of morphological keys.8-10 A total count was then made of the worms recovered from the gastrointestinal tract.

Analytical Methods: The different species of parasites present in an intermediate or final host, constitute an infracommunity.11

The habitat term refers to the host of the gastrointestinal nematode infracommunity.12 The terms prevalence, expressed as a percentage (number of host infected with a particular parasite species divided by the number of hosts examined), mean intensity (mean number of individuals of a particular parasite species per infected host in a sample) and abundance (mean number of individuals of a particular parasite species per host examined in a sample, including uninfected hosts) follow the definitions of Margolis et al.13

The index of the evenness and its associated diversity measure was calculated.14 Evenness (E) is defined as the degree to which the abundance is equal among the species present in a sample or community.15 The degree of overlap of the real community distribution and this theoretical one is an adequate measure of the evenness of the sample. In the present paper the Shoerner's niche overlap index was used.14

The effective number of species is a measure of the degree to which proportional abundances are distributed among the species and the diversity index (D) is a measure of the species composition in a community in relation to the total number of species or species richness (S) and evenness (D = S x E). It is more suitable than other diversity indices because its units are the number of species and it measures the effective number of species presents in a sample.14

The structure and composition of parasite communities between both dry and rainy periods were compared by means of Spearman's rank correlation coefficient and the Sorensen similarity index respectively.16, 17

The Bulla diversity index was 1.94 and 1.66 species for dry and rainfall periods respectivelly and the evenness index was 0.216 for dry and 0.185 for rainfall periods. The community dominance index was 91.8% for dry and 72.5% for rainy periods (Table 2).

The similarity of parasite infracommunities in both periods was high (Css = 77%) but the structure was dissimilar (rs = -0.08).

Necropsies of sheep from abattoirs can be of interest, particularly in developing countries, because the animals killed in a slaughterhouse from a small region, present an image of the parasitism in that region at the particular period of the year.2, 5

All species of helminths recovered and identified in the present study have been reported by Díaz18 except C. fuelleborni, which was reported in Venezuela by others.19

Many factors are responsible for the fluctuations in number and availability of infective larvae of strongyles and the transmission is a complex process which extends from eggs hatching to survival of infective third larvae to be ingested by a grazing susceptible host.20

The key factors governing infective parasite populations are temperature and moisture in the external environment.21 Most strongylides are capable of developing and maintaining populations of infective larvae over considerable ranges of temperature and moisture.22 On the other hand parasites respond differently to the same environmental variables during different phases of their life cycle.22

The differences observed between the dry and rainy periods might be explained in terms of the:

- increase of the host susceptibility to the parasitic infection, because of deficient nutritional conditions prevalent during dry period.

- existence of microhabitats containing enough moisture to allow the grass to growth in this places, the concentration of animals and therefore of feces is frequent and may produce an increment of the risk of parasitic infection.12

The sinecological analysis showed that the composition of the parasitic community observed in both periods was similar, but the structure was different, because of the numerical importance of parasitic species had a different pattern.

A community has a high evenness and species diversity indices if many species are abundant. This is because both indices have two components: the number of species and the relative abundance of each species.1, 14, 16, 17

The evenness and diversity indices had low values, as a result of the numerical dominance of only two genera Haemonchus and Trichostrongylus and three species (H. contortus, T. axei and T. colubriformis) and the unequal numerical repartition of species within the community during both periods.

These results suggest that in arid regions, the complete life cycle of strongylides, principally H. contortus and Trichostrongylus sp might be achieved and therefore eggs and infective larvae of both parasites seem to resist dessication better than those of the other genera. However, studies using tracer lambs with worm burden examination at necropsy are required to confirm these results.

11.- AHO J, BUSH A. Community richness in parasites of some fresh water fishes from North America. In: species diversity in ecological communities: historical and geographical perspectives. Edit. by R. Ricklefs and D. Schluter. The University of Chicago Press, USA, 1993; 185-93. [ Links ]

Ackowledgements. The authors wish to thank R. Nino Incani and David Ammons from the Fac. of Medicine, University of Carabobo, Venezuela and School of Veterinary Medicine, The University of the West Indies, Trinidad, respectively for theirs helpful comments and assistance with the English translation of this manuscript. This work was supported by a grant from FONAIAP/ Venezuela.